[Step #1]
One way synchronize users from my custom built password-protected system into vBulletin. This means that I do not allow any registrations on vB. Users register on my own customized system. Then on first login, vB user is automatically created. On following logins, if user already exists, my script simply updated vB user table off of my user table.
Note #1: this scenario presumes that usernames and passwords on your system and on vB are ideantical.
Note #2: 'salt' for password encryption is not used, i.e. it's an empty string.
[Step #2]
I used vB login script to create somewhat of a hybrid of mine. This script is available for your use at your own risk [script attached]
Basically, you are blindly assigning cookies and session upon successful login.
Hope this helps to someone, it did help me a great deal as well as my audience. Before, many folks refused to enter username/password twice, not to mention registering twice.
Update: tested with Pro@vbulletin.org on his vBulletin v. 3.5, and concluded that it won't work well with that. Session table requires some more information.
Did this ever work out?
It did and still is working on my vB 3.0.7. I have worked with a couple of guys who have 3.5 and the change made in authentication process of 3.5 doesn't allow me to force login my way. I hope someone else manages to figure newer versions as I am running some newer forums and would like to have it.